In this application, we will take advantage of the existing data of plasma lipid markers in 28,345 Women's Health Study (WHS) participants and conduct a detailed, prospective cohort evaluation of standard lipid markers (total cholesterol, low-density lipoprotein cholesterol (LDL-C), HDL-C, triglycerides), and novel lipid markers (apolipoprotein A-1 (Apo A-1) and Apo B100), as well as long-term use of statins in relation to risk of total invasive cancer and breast, colorectal and endometrial cancer. Blood lipids may affect cancer risk through their association with insulin resistance, inflammation and oxidative stress. Statins have anti- inflammatory, proapoptotic, and antiproliferative effects, which may make them relevant to cancer prevention. Also, we will evaluate these associations by factors that affect plasma lipid levels (e.g., menopausal status, body mass index, physical activity, postmenopausal hormone use, alcohol intake, and other plasma obesity- related markers), as well as by tumor characteristics (e.g., estrogen receptor status). We will also address the issue that the preclinical disease may alter blood lipid levels by conducting an analysis of excluding the first 2 years of follow-up. By June 2011, with a mean follow-up duration of 17 years, we expect 3,506 confirmed incident invasive cancers, including 1,473 breast cancers, 338 colorectal cancers, and 304 endometrial cancers in 28,345 women who provided a baseline blood sample and were free of cancer and cardiovascular disease at baseline. Standard and novel lipid markers were already measured in these 28,345 blood samples. Thus, this proposed study affords a unique and extraordinary opportunity to conduct a prospective cohort analysis of plasma lipid markers and cancer risk. This proposed study is innovative as the associations between plasma Apo A-1 and Apo B100 and cancer risk are largely unexplored in prospective studies. Few studies have examined blood lipid markers and long-term statin use in relation to endometrial cancer risk. This proposed study will be the first evaluation of the relations between plasma Apo A-1 and Apo B100 and overall cancer risk and between Apo B100 and endometrial cancer risk, as well as the first prospective evaluation of Apo A-I and risk of breast and endometrial cancer. This study also has numerous other strengths, including the prospective cohort study design, large sample size (n = 28,345), large numbers of cases (n = 3,506), long duration of follow-up (17 years), excellent environment, availability of other plasma obesity-related markers, detailed information on covariates and tumor characteristics. The ongoing WHS provides follow-up, ascertainment and documentation of cancer cases, and data on biomarkers, statin use, and covariates. Thus, this proposed study affords a unique opportunity to test promising hypotheses about cancer risk in an exceptionally cost-efficient manner. Findings from this study will help clarify the role of plasma lipid markers in identifying women at high risk of cancer who would most benefit from chemoprevention or increased screening and the role of long-term use of statins in cancer chemoprevention.